CD4 is a surface molecule of human helper T-lymphocytes that serves as the receptor for human immunodeficiency virus (HIV), the causative agent of AIDS. We have focused on two areas related to CD4 and HIV: 1) Structure/function studies of the interaction of CD4 with the HIV-1 envelope glycoprotein (gp120/gp41 complex). The focus of this work is to identify structural changes in the envelope glycoprotein which occur upon CD4 binding, and which are involved in virion fusion with the cell membrane. Soluble CD4 was found to promote specific dissociation of gp120 from the envelope glycoprotein complex. Studies with synthetic peptide derivatives suggested that a particular region of CD4 (the CDR3 region) plays a critical role in inducing this structural change. Future efforts will focus on detailed analysis of the sites on CD4 and the envelope glycoprotein involved in fusion (using site-directed mutagenesis coupled with refined assays to measure membrane fusion)> We have also made progress in developing approaches to detect conformational changes leading to exposure of the presumed fusogenic regions of the envelope glycoprotein. 2) Design of CD4-based therapeutics for the treatment individuals infected with HIV. We have continued to study a genetically engineered hybrid toxin (CD4-PE40) which is targeted to selectively kill HIV-infected cells. Previous work demonstrated potent anti-HIV activity against infected T- cells and monocyte/marcophages (primary cultures and continuous cell lines); the toxin was also shown to have highly synergistic effects with reverse transcriptase inhibitors. We have now found that CD4-PE40 is active against cells expressing diverse forms of HIV and SIV envelope glycoproteins, which differ in extent of processing, presence of a cytoplasmic tail, and binding affinity for CD4. These results suggest that the activity of CD4-PE40 may not be compromised in vivo by normal variations in envelope glycoprotein structure. We are now developing assays to test the activity of the hybrid toxin against primary isolates of HIV, including variants which have been shown to be highly refractory to neutralization by soluble CD4. We will also test newly developed CD4-PE40 constructs for possible enhanced anti-HIV activity.